This application relates generally to optical lensing and more specifically to techniques and devices for routing optical signals.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called an optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80-channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Opto-mechanical switches, which offer acceptable optical specifications, are too big, expensive and unreliable for widespread deployment. New integrated solid-state technologies based on new materials are being researched, but are still far from commercial application.
Consequently, the industry is aggressively searching for an all-optical wavelength routing solution that enables cost-effective and reliable implementation of high-wavelength-count systems.
Embodiments of the invention are directed to an optical arrangement and method for receiving a light beam having a plurality of spectral bands and directing subsets of the spectral bands along optical paths to respective optical elements. The light beam is received at an input port. The optical elements which route the spectral bands are configured as a substantially planar array. A dispersive element is configured to angularly spread the light beam, after it has been collimated, into a plurality of angularly separated beams that correspond to the plurality of spectral bands. A first focusing element is disposed with respect to the dispersive element and with respect to the array of optical elements such that variation of focal length with wavelength of the separated beams is compensated by the field curvature of the optical system, and the final image surface is flattened. Different embodiments are adapted for positive and negative field curvature aberrations.
In certain embodiments, the dispersive element is a reflective diffraction grating. The first focusing element may be further disposed with respect to the reflective diffraction grating to collimate the light beam before the light beam encounters the reflective diffraction grating. In such embodiments, the first focusing element may be, for example, a lens disposed between the input port and the reflective diffraction grating or a curved mirror disposed to intercept light from the input port. In one embodiment, the input port is substantially coplanar with the array of optical elements. For a positive field curvature aberration, the input port may thus be positioned proximate the optical element corresponding to the shortest-wavelength spectral band, with optical elements corresponding to progressively longer-wavelength spectral bands positioned progressively farther from the input port. For a negative field curvature aberration, the input port may be positioned proximate the optical element corresponding to the longest-wavelength spectral band, with optical elements corresponding to progressively shorter-wavelength spectral bands positioned progressively farther from the input port.
In other embodiments, the dispersive element is a transmissive diffraction grating. A second focusing element is disposed with respect to the transmissive diffraction grating to collimate the light beam before the light beam encounters the transmissive diffraction grating. The first and second focusing elements may have a common symmetry axis that is substantially orthogonal to the array of optical elements. The input port may be positioned within a plane parallel to the array of optical elements. For a positive field curvature aberration, the input port may be displaced from the symmetry axis by an amount substantially equal to a displacement from the symmetry axis by the optical element corresponding to the shortest-wavelength spectral band; the optical elements corresponding to progressively longer-wavelength spectral bands may thus be progressively farther from the optical element corresponding to the shortest-wavelength spectral band. For a negative field curvature aberration, the input port may instead be displaced from the symmetry axis by an amount substantially equal to a displacement from the symmetry axis by the optical element corresponding to the longest-wavelength spectral band; the optical elements corresponding to progressively shorter-wavelength spectral bands may thus be progressively farther from the optical element corresponding to the longest-wavelength spectral band. In one embodiment, the first focusing element is a lens disposed between the transmissive diffraction grating and the array of optical elements and the second focusing element is a lens disposed between the input port and the transmissive diffraction grating.
In still other embodiments, the dispersive element is a prism. A second focusing element may be disposed with respect to the prism to collimate the light beam before the light beam encounters the prism. Alternatively, the dispersive element may be a grism.
The array of optical elements may comprise an array of routing elements. In one embodiment, each such routing element is dynamically configurable to direct a given angularly separated beam to different output ports depending on its state. In alternative embodiments, the array of optical elements comprises an array of detector elements.
In certain embodiments, the dispersive element is angularly positioned with respect to the first focusing element to minimize the field curvature aberration. In other embodiments, the first focusing element is designed to have a specific field curvature aberration based on an angular position of the dispersive element with respect to the first focusing element.
Embodiments of the invention may be incorporated in a wavelength router. Such a wavelength router is configured to receive, at an input port, light having a plurality of spectral bands and to direct subsets of the spectral bands to respective ones of a plurality of output ports. A routing mechanism is provided having a substantially planar array of dynamically configurable routing elements, each of which is structured to direct a given spectral band to different output ports, depending on its state. A free-space optical train included a dispersive element and focusing element forming an optical arrangement as described above is disposed between the input port and the output ports to provide optical paths for routing the spectral bands.